class_docstr: A subgraph of a TensorFlow computational graph.
  
  TensorFlow graphs are often too low-level to represent our conceptual
  understanding of a model
  
  OverlayGraph can be used to represent a simplified version of a TensorFlow
  graph. Each node corresponds to a node in the original TensorFlow graph, and
  edges correspond to paths through the original graph.
code: def __getitem__(self, index):
      return self.name_map[OverlayNode.as_name(index)]
  
imports: from collections import defaultdict
  import numpy
  import tensorflow
label_desc: Use image node_id 2 for calling the OverlayGraph obj's underlying member method code with example usage: obj.__getitem__(index) and returns: self